The best board length is the shortest stock length that covers the full run in one piece, with a little left over for squaring the ends and any overhang. When no single length covers the run, pick a combination whose seams land on joists and can be staggered between rows, or split the deck with a breaker board. The examples below show how to compare the options on actual decks.
Know what lengths you can actually buy
Before you plan anything, find out which lengths your supplier stocks for the board you want.
- Composite and PVC. The major US brands sell decking mainly in 12, 16 and 20 ft lengths. Trex Transcend and TimberTech’s grooved Legacy boards come in all three. There are exceptions: TimberTech’s square-shoulder Legacy boards are listed only in 16 and 20 ft, and Cali Bamboo’s BamDeck is sold in 16 ft.
- Pressure-treated and cedar. Dimensional lumber is generally stocked in 2 ft increments, usually 8 to 16 ft, and some yards carry longer pieces. Selection varies a lot by store and region, so check before you design around a 20 ft board.
A local store may carry only one or two of the lengths a brand makes, even though the brand makes all three. Special orders can add lead time. Settle this first, because it limits every layout decision that follows.
The ideal: one board per row
A run is the length of the deck in the direction the boards go. If your run is shorter than a stock length, each row is one board with no seams. That’s the best result: no butt joints to gap, no doubled joists, and a clean look from end to end.
Example: 20 × 12 ft deck, boards running 20 ft. With 5-1/2” boards and a 3/16” gap you need 26 rows (see the deck board spacing guide for that math). If the decking overhangs the rim or fascia, the run is a little over 20 ft, so check whether 20 ft boards are long enough. Many installers trim the factory end square before fastening. TimberTech’s guide says to check and cut factory ends so they’re square. A 20 ft board with no length to spare can come up short once both ends are trimmed and you add an overhang.
So:
- If the boards run 20 ft on a deck framed to exactly 20 ft with no overhang, 20 ft boards usually work.
- If the deck is 20 ft plus a 1” overhang at each end, measure your boards. If they aren’t long enough, plan for seams or a breaker board, or shorten the framing slightly.
This is the single biggest reason to settle board length before the framing goes up. Taking 2” off a frame costs nothing at the design stage. Finding out at the last row that every board is 1/2” short is expensive.
When the run is longer than your longest board
Here you have three options.
Option 1: Butt joints over doubled joists
The ends of two boards meet over a joist. Manufacturers and wood associations agree that each board end needs its own support:
- TimberTech says board ends must fall on a joist and to always double joists at butt seams.
- The Southern Forest Products Association says best practice is to double the joists to give each board end a full 1-1/2” bearing.
- Fiberon recommends adding 2x blocking at butt joints in areas with extreme temperature swings.
That means every seam location needs a doubled joist or blocking, so plan seams before you build the frame. Butt joints in composite also need the temperature-based end gap from the installation guide.
Option 2: Staggered seams
Trex’s joist guide recommends staggering seams so they don’t all line up on the same joist. A single line of seams across the deck looks like a mistake, and it concentrates movement on one framing line. Staggering spreads the joints out, but it means you need doubled joists or blocking at several locations instead of one.
A common approach is to alternate two patterns, row by row:
- Row A: long board, then short board
- Row B: short board, then long board
This puts seams at two joist lines and repeats every other row. Some builders prefer a three-row pattern because an A-B-A-B repeat can still read as a pattern from a distance. Either way, make sure the seams in adjacent rows are separated by at least one full joist bay.
Option 3: A breaker board
A breaker board (also called a divider or parting board) runs across the deck, perpendicular to the field, and every row butts into it. It replaces scattered seams with one deliberate line. It’s often the cleanest answer for runs longer than 20 ft. It has its own framing requirements, covered in the breaker board guide.
Worked example: a 24 ft run
Take a 24 × 12 ft deck with boards running 24 ft, 26 rows, and joists at 16” on center starting at one end. Ignore overhang for now and compare three plans:
| Plan | Boards per two rows | Seams | Linear ft bought | Linear ft used | Waste |
|---|---|---|---|---|---|
| 12 + 12, all seams at 12 ft | four 12 ft | one aligned line | 624 | 624 | ~0%, but seams align |
| 16 + 8 alternating with 8 + 16 | two 16 ft + one 16 ft cut in half | two staggered lines | 624 | 624 | ~0% before trimming |
| 20 + 4 alternating with 4 + 20 | two 20 ft + two 4 ft pieces (three per 12 ft board) | two staggered lines | 26 × 20 + 9 × 12 = 628 | 624 | small |
Check that the seams hit joists. At 16” spacing, joists sit at 0”, 16”, 32” and so on.
- 12 ft = 144” = joist 9. That lands on a joist.
- 16 ft = 192” = joist 12, and 8 ft = 96” = joist 6. Both land.
- 20 ft = 240” = joist 15, and 4 ft = 48” = joist 3. Both land.
On paper, all three plans work with almost no waste. In practice the plans differ in other ways:
- Cutting a 16 ft board into two 8 ft pieces leaves no allowance for the saw kerf and for squaring the factory ends. The pieces will be slightly short of 96”. Real plans need a few inches of slack, which pushes you up a length or means trimming the framing to suit.
- The 20 + 4 plan uses short 4 ft pieces near the edge of the deck. Short pieces show every gap and are more fiddly to fasten.
- Aligned seams at 12 ft would look much better as a breaker board, since you’re paying for the doubled joist line anyway.
The lesson is that a plan with zero waste on paper rarely stays zero once you allow for trimming. Leave room for it.
Worked example: an awkward 14 ft run
A 14 ft run (168”) with 26 rows:
- 16 ft boards. Each row wastes about 2 ft, so 26 × 2 = 52 linear ft of offcuts out of 416 bought, about 12.5%. You get no seams at all.
- 12 ft + 2 ft pieces. 144” lands on joist 9, and the 24” pieces can be cut from leftovers. Waste is low, but you get a seam on every row near one edge, plus doubled joists.
Most builders would buy the 16 ft boards here. The offcuts aren’t entirely wasted either. Two-foot pieces are useful for stair treads, blocking or a step at a door. This is why a cut list matters: it shows where offcuts can be reused instead of thrown away.
How length affects cost
Price per linear foot isn’t always the same across lengths. Some suppliers charge more per foot for the longest boards, and some price them about the same, so get prices for the lengths you’re actually comparing. Longer boards also cost more to deliver, and handling a 20 ft composite board on your own is awkward.
To compare options, multiply the linear feet bought for each plan by that length’s per-foot price. Then add the cost of the extra framing that seams need. A plan with less waste can still cost more if it adds a line of doubled joists. The deck cost calculator puts board, framing and fastener costs side by side so you can compare plans.
A short checklist before you order
- Confirm which lengths your supplier stocks for your chosen board.
- Measure the true run, including any overhang at both ends.
- If one stock length covers it with a few inches to spare, use it.
- If not, decide between staggered seams and a breaker board, and draw the seam locations on the framing plan.
- Confirm every seam lands on a doubled joist or blocking.
- Add a waste allowance. TimberTech suggests about 10% scrap for a standard layout. Patterns with mitres or angles generally need more.
The deck board calculator has a board-length optimizer that checks 12, 16 and 20 ft combinations against your run and joist spacing. It also produces a cut list showing each row’s pieces and where the offcuts go. If you’re still at the framing stage, the deck framing calculator helps you place the extra joists that seams need.
Frequently asked questions
Is it better to use longer deck boards?
Longer boards mean fewer seams, which usually looks better and saves framing. They only make sense if they fit the run without a lot of offcut, though. A 20 ft board on a 14 ft run wastes 6 ft per row. Match the length to the run first, then minimize seams.
Do butt joints in decking have to land on a joist?
Yes. Every manufacturer guide we’ve reviewed requires board ends to be supported, and most call for doubled joists or blocking so each end gets its own bearing and fasteners. A board end that floats between joists will sag and can break.
How far apart should butt joints be staggered?
Manufacturers mainly say not to line seams up on the same joist. A practical rule is to separate seams in adjacent rows by at least one joist bay and to avoid an obvious repeating pattern. If seams are unavoidable over a large area, a breaker board often looks better.
Can I use deck board offcuts?
Yes. Short offcuts often work for stair treads, a landing step, short rows at corners, or blocking under a picture frame where the material is suitable. Planning the cut list before you buy is the easiest way to make sure offcuts get used.